It has long been known that the heart muscle provides its pumping function in response to electrical events which occur in the atrium and ventricle of the heart. Conductive tissue connects the atrium and the ventricle and provides a path for electrical signals between the two areas. In a normal heart, a natural atrial event spontaneously occurs in the atrium and a corresponding ventricular event occurs later in the ventricle after a time interval typically called the A-V interval. After the ventricular event a new atrial event occurs in the atrium to trigger a succeeding ventricular event. The synchronized electrical events occurring naturally in the atrium and ventricle cause the heart muscle to rhythmically expand and contract and thereby pump blood throughout the body.
In a diseased heart, atrial and ventricular events may not naturally occur in the required synchronized manner and the pumping action of the heart is therefore irregular and ineffective to provide the required circulation of blood within the body. The required synchronized activity of such diseased hearts can be maintained by a pacemaker which applies synchronized stimulating pulses to either the atrium or the ventricle or both.
In the early stages of pacemaker development, pacemakers were employed to asynchronously stimulate the ventricle of the heart without regard to natural electrical activity occurring in either the atrium or the ventricle. Although this approach had the advantage of simplicity, there was considerable risk because paced ventricular events could interact with natural ventricular events to cause a dangerous arrhythmia.
As the art of pacing advanced, pacemakers were provided with circuitry which sensed the occurrence of natural ventricular and atrial activity and paced the heart in either the atrium or ventricle only when required to maintain proper operation of the heart. A dual chamber pacemaker can operate in what is known as DDD mode, wherein electrical events are sensed in the atrium and in the ventricle and the atrium and ventricle are paced accordingly. Pacemakers may also be operated in VDD mode to sense electrical events in the atrium and ventricle but pace only in the ventricle. Other pacemaker modes of operation are employed to sense or pace in either the atrium or the ventricle, as required for the particular needs of a patient.
The effectiveness of the heart as a pump is dependent both on the heart rate and on coordination between the upper and lower chambers of the heart or synchrony between the atrium and the ventricle. When the atrium and the ventricle are coordinated, the heart is relatively more efficient and, therefore, more effective as a pump. If the two chambers are only marginally coordinated, a higher minimum heart rate for the ventricle may be necessary in order to maintain a desired flow. If the two chambers are coordinated, on the other hand, a somewhat lower minimum heart rate can be tolerated.
Prior art VDD pacemakers have not responded adequately to a situation wherein the heart rate is slow but can be synchronized. In general, minimum ventricular heart rates have been set, for example, at a rate of 60 beats per minute. Whether or not synchrony between the atrium and the ventricle was maintained, if the heart rate in the ventricle should fall below the predetermined minimum, the pacemaker would stimulate the heart at the predetermined rate. This could result in a loss of synchrony and a corresponding loss in cardiac efficiency. It is an object of our invention, therefore, to provide a VDD cardiac pacemaker with extended atrial sensing and VDD hysteresis to accommodate synchronous pacing at lower cardiac rates.
It also an object of our invention to provide a VDD pacemaker which provided two minimum rates: a first minimum rate in the absence of synchrony and a second minimum rate, lower than the first minimum, which can be maintained if synchrony is achieved.
For a cardiac pacemaker provided with two minimum rates, as suggested above, it is desirable for the pacemaker to maintain synchrony whenever possible. If the pacemaker, maintaining synchrony, passed the first minimum rate to the absolute second rate, synchrony would be lost. When synchrony was lost, adequate blood flow would only be maintained at the first, relatively higher minimum rate. The pacemaker might jump from the second, lower minimum rate to the first, higher minimum rate. If synchrony can be reestablished within a few cardiac cycles, however, the pacemaker should be able to pace below the first minimum rate. It is an important object of our invention, therefore, to provide a cardiac pacemaker which attempts to provide the capacity to reestablish synchrony in each cardiac cycle. It is also an object of our invention to reestablish synchrony at rates below the first, asynchronous minimum rate, under appropriate circumstances.
Synchrony is important not only at low cardiac rates, but also at high cardiac rates. In rate responsive VDD pacemakers, pacing the ventricle in response to the atrium allows the pacemaker to respond in a physiologic manner. It is possible, however, for the natural pacemaker of the atrium to malfunction at high rates. If this situation occurs, the atrial heart rate may slow even though there is a continuing physiologic need for increased cardiac output and elevated heart rate. Tracking the atrium in such a situation can result in an inappropriate reduction in the ventricular rate. This undesirable situation can be avoided by providing that the minimum asynchronous heart rate be adjustable and dependent upon a sensor correlated to physiologic needs. Such a sensor might be an accelerometer, a thermistor, or an oxygen or pH sensor, as examples. Consequently, an important object of our invention is to provide a VDD pacemaker with an adaptable asynchronous minimum heart rate.